For a positive electrode current collector of a stationary valve regulated lead-acid battery (hereinafter, sometimes referred to as “VRLA battery”) to be used in an emergency power supply unit (UPS) in a hospital, a factory or the like, generally a cast grid is used, and an expanded grid is not used. Usually, a stationary VRLA battery is float-charged and kept in a state of full charge at all times, and therefore generally, corrosion of a grid in a positive electrode dictates a life mode. For imparting corrosion resistance, a crosspiece of a positive electrode grid is required to have a certain thickness, but in the case of an expanded grid, it is difficult to prepare a thick grid, and thus the expanded grid is not suitable for a large stationary VRLA battery. When an expanded grid is used as a positive electrode grid of a VRLA battery, a positive electrode corrosion current during charge is large, so that a float current is not used for generation of oxygen, and therefore the oxygen absorption cycle in a negative electrode plate becomes incomplete. When the oxygen absorption cycle becomes incomplete, the negative electrode is polarized, and as a result, the float current decreases, so that the positive electrode comes into a poor charged state, leading to a reduction in discharge capacity. When an expanded grid is used in a positive electrode of a VRLA battery that is float-charged, the float current decreases, so that the battery comes into a poor charged state, leading to a reduction in life. For this reason, cast grids have been used in valve regulated lead-acid batteries.
Related prior arts will be shown. Patent Document 1 (JP-A-2006-294296) discloses that rolling is performed at a low rolling reduction ratio in an initial stage of rolling to maintain the strength of the deep part of a rolled sheet. Patent Document 2 (JP-A-2008-84676) describes that for an expanded grid (positive electrode grid) in a lead-acid battery, grain boundary corrosion is suppressed when the average particle size along a rolling direction is 150 μm or less. Patent Document 3 (JP-A-2000-348758) discloses a VRLA battery including an expanded grid as a positive electrode grid.